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Bridge design stretched to the limits: Cable-stayed bridges are a type of suspended crossing. Spare but functional, they exploit a form of bridge engineering that is bringing down the cost of progressively longer crossings

By BILL O'NEILL

Engineers are stretching their expertise to design and build long bridges
more cheaply and quickly than ever before. Next week, barely six years after
it was first thought of, one of Europe’s longest bridges opens across the
River Thames at Dartford, about 30 kilometres downstream of the City of
London.

The Dartford River Crossing, which is designed to relieve a bottleneck
on London’s orbital motorway, the M25, puts the first major road bridge
over the Thames since Victorian engineers completed Tower Bridge in 1894.
The bridge accounts for about three-quarters of the cost of the crossing,
which includes two long viaducts, stretches almost 3 kilometres and has
a price tag of just £120 million.

The crossing is also the first major highway in Britain to be privatised.
This factor alone, its owner claims, ensured the development of the most
economical structure in the shortest possible time. Going by similar advances
elsewhere in the world, however, technical expertise rather than commercial
nous seems the more significant factor. Public authorities in continental
Europe, North America and Japan are proving equally adept at getting more
value for money from their bridge engineers.

At the heart of this revolution in bridge engineering is a class of
structure, known as the cable-stayed bridge, that has long been the poor
cousin of the traditional design for lengthy spans, the suspension bridge.
Both are forms of suspended crossing, relying on cables above them for support
rather than columns below. Although suspended crossings – and suspension
bridges in particular – are more complicated to design and build than bridges
supported on columns, they can provide long, uninterrupted …